An electrical machine like a generator contains a number of permanent magnets, which interact with at least one coil to generate electrical power. For the magnets used a compromise needs to be found. It is necessary to minimize or even avoid at least some of the following problems:
First of all the magnetic force (magnetic field-strength) of the magnets will vary due to their individual characteristics and tolerances. Periodical torque pulsations will occur if the machine is within the status “no-load”, “idling” or “full load”.
Next the number and/or the size of used permanent magnets needs to be minimised due to the steadily increasing costs.
The torque stated above is denoted as “cogging torque” if the machine is in “no-load”-status, while it is denoted as “ripple torque” if the machine is “load”-status.
The torque pulsations may result in vibrations, which propagates inside the machine and within a used supporting structure of the machine also. The torque pulsations may harm mechanical and electrical components.
Furthermore the torque pulsations may generate acoustic noise with low frequencies. The frequencies are audible and thus disturb the environment, the human-beings and the wildlife.
Especially if a huge direct drive generator is within a wind turbine the disturbance needs to be reduced or even avoided.
Several techniques are known to reduce “cogging torque” or “ripple torque”. For example the permanent magnets are shaped specifically or so called “dummy slots” are used inside the electrical machine.
The magnet shaping is advantageous for a given current and a required torque. The magnet shaping can be done in regard to minimize the amount of magnet material needed.
It is also possible to reduce cogging-torque and/or ripple-torque by an optimized shaping of the magnets.
A huge number of optimized magnet-shapes is known in the prior art.
An important and commonly applied one shows upper magnet corners, which are cut away. This is called chamfering. The chamfer angle used may be 45° but also alternative chamfer angles are known in the prior art. However this kind of chamfering does not reduce the cogging-torque and the ripple-torque to a satisfactory level.
Document EP 1 076 921 A1 describes a magnet piece with a cross sectional geometry. The geometry corresponds to the half-cycle arc of a sine curve. It is very difficult and expensive to manufacture this geometry. Even this approximation does not reduce the cogging-torque and the ripple-torque to a satisfactory level.